With the increase in the popularity of selective call messaging, channel capacity for selective call systems has become a scarce commodity in major metropolitan areas. This popularity has resulted in long delays between the input of selective call messages to a selective call terminal and the transmission of the selective call messages from the terminal. As new selective call services are introduced on existing channels, the overcrowding of existing selective call channels is expected to increase. One solution to this problem is to increase the number of channels allocated to selective call messaging. This solution can only be implemented by the government regulating authorities who are already overburdened with requests for more RF channel allocations from other types of services, for example, land mobile and cellular telephone. Even if new channels are made available by the government, there is no guarantee that a particularly busy selective call system provider will be able to obtain a license on the new channel.
Another solution to the overcrowding is to increase the amount of traffic that can be handled on the existing channels by increasing the baud rate (i.e. the number of bits transmitted per second (bps)) of the transmitted signal. This solution has been implemented in the United Kingdom where the bit rate for a selective call signalling protocol identified as Radio Paging Code No. 1 of the Post Office Code Standardisation Advisory Group (POCSAG) has been increased from 512 bps to 1200 bps. Unfortunately, simply introducing new 1200 baud selective call receivers onto an existing selective call system's channel does not substantially increase channel capacity unless the older 512 baud units are retired from service and replaced with 1200 baud pagers. In addition, merely increasing the baud rate, without modifying the code format, has a number of undesirable effects. For example, for each doubling of the bit rate, the paging sensitivity in the Gaussian environment degrades by two to three decibels (dB). Also, increasing the bit rate generally requires the decoder in the selective call receiver to run faster resulting in a decrease in battery life. Finally, for each doubling of the bit rate in a Rayleigh fading environment, the maximum fade length that can be tolerated is reduced by one half which may result in the loss of six dB or more of paging sensitivity in the fading environment. This loss of sensitivity in the fading environment is caused by an increase in the number of erroneous bits received by the selective call receiver due to the fact that the burst errors at the higher baud rate affect more bits. Most signalling protocols have error correction algorithms which can reconstruct the information transmitted as long as the number of erroneous bits received is below a predetermined number. When the erroneous bits received increases above the predetermined number allowed, the information received cannot be reliably reconstructed.
The loss in Gaussian noise sensitivity is a cost of increasing the bit rate. The loss in fade protection, however, can be overcome through the use of bit interleaving. For instance, in the Golay Sequential Code (GSC), an alternate selective call signalling protocol to POCSAG, the message information consists of eight (15,7) BCH code words interleaved to a depth of eight and transmitted at 600 baud. This provides sixteen bits of burst error protection which is equivalent to 27 msec of fade protection. To provide the same amount of fade protection at 1200 baud requires the interleaving depth to be increased to sixteen. However, increasing the interleave depth generally complicates the selective call receiver decoder since more memory (RAM) is required for the deinterleaver. Furthermore, if an attempt is made to make the decoder adaptive to a variety of bit rates while maintaining a constant amount of fade protection, the deinterleaver must be reconfigured with each change in the bit rate.
One implementation of signal interleaving at different transmission speeds is disclosed in European Patent Application 88-106961/16, published as European Patent Office Patent Publication No. 261-205-A (EPA '205). The system disclosed in EPA '205 accommodates receivers of different bit rates without reconfiguring the deinterleaver with each change in the bit rate and is resistive to burst errors even if the bit rate is increased. The EPA '205 system, though, requires the selective call receiver decoder to run at higher operating speeds for higher baud rates resulting in reduced battery efficiency and shorter battery life for the selective call receiver's battery.
Thus, what is needed is a method and apparatus for interleaving and deinterleaving a signal in a selective call system environment at successively deeper interleaving depths with successively higher transmission baud rates wherein the battery life in the selective call receivers is not decreased due to the higher baud rate of transmission and the loss in paging sensitivity is minimized and the maximum tolerable fade length are not decreased.